As is well known, a number of chemicals are used as additives in fluids delivered to subterranean locations. For instance a fluid used for hydraulic fracturing might contain cross linkers, biocides and/or scale inhibitors. Water which is injected into a subterranean reservoir to bring about secondary oil recovery might include biocides and/or corrosion inhibitors.
It is well known to carry a chemical additive to a location below ground by dissolving the additive in a carrier fluid. Of course this possibility is not available if the additive which it is desired to use (eg an additive having desirable functionality combined with an economical cost) is insoluble in the carrier fluid. Thus water which is being injected below ground is an unsuitable carrier for hydrophobic additives. Of course it is well known to convey solids as a suspension in a thickened liquid, as is commonly done when carrying out hydraulic fracturing, but the carrying liquid then needs to be designed as a thickened carrier and the energy required for pumping is greatly magnified.
Of relevance to some forms of the invention disclosed below is the use of tracers in water which is injected into a subterranean location. Detection of tracer in water which is subsequently produced from below ground may provide some information about paths of flow below ground and some information about residence time of water in its passage below ground from the injection well to the production well. However, there is at present no technology available which will show whether the injected water makes contact with oil during passage through the reservoir.
Of relevance to some other forms of this invention is the crosslinking of thickening polymers to further increase viscosity. The ability to delay cross-linking has always been a highly desirable property for a number of wellbore service fluids, notably including fracturing fluids. For example in fracturing, the ideal scenario is that the fluid has low viscosity while it is at the surface and whilst it is being pumped down the wellbore, after which the viscosity increases. This would result in a reduction in the overall energy requirements for pumping due to the low viscosity whilst the fluid is in transit at the surface and within the wellbore.
A number of systems are known where a fluid includes a thickening polymer, and a cross linking agent is used to bring about cross linking. Ideally, the action of the cross linking agent is delayed until the fluid has been pumped to a downhole location. Polysaccharides, especially guar are commonly used as thickening polymers. Borate is a common cross linker for polysaccharides but it is also known to use other cross-linking agents including zirconates and titanates. Chromium ions can be used as cross-linkers, especially of polyacrylamides.
One system which can be used to cross-link a variety of polymers which contain hydroxyl, amino, amide or even thiol groups, especially polyacrylamides, is a combination of an aldehyde and a phenolic compound as for example discussed in U.S. Pat. No. 4,485,875 which names formaldehyde, acetaldehyde and glyoxal (a dialdehyde) with a preference for formaldehyde. Aldehydes alone can bring about crosslinking, but incorporation of phenol into the cross-linking system gives a highly activated cross-linking agent and, in the presence of polyacrylamide, results in the formation of gelled fluids with thermal stability at elevated temperatures (i.e. above 90° C.). Unfortunately the deployment of phenol is problematic. Phenol is toxic, causes burns and is hygroscopic. Furthermore, the use of phenol precursors which would mitigate these risks, such as phenyl acetate (PhOAc) or salicylate (PhSal), mentioned in U.S. Pat. No. 5,447,986 for instance, is problematic due to both of these compounds being poorly water soluble. The phenol precursors float upwards in water before they undergo hydrolysis to generate the phenol. A concentration gradient is thus generated comprising a high concentration of phenol at the top of the solution and virtually none at the bottom. As a result, the gel which is formed is therefore non-homogeneous, being ‘over cross-linked’ at the top (resulting in syneresis) and ‘under cross-linked’ at the bottom (weak, poorly viscous gel).